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Technical Support of container desiccant

Source:Globaldry desiccant     2012-09-19

 

1、What is the relative humidity? How to calculate the water content by relative humidity
1.) Relative humidity is the percentage of current water vapor pressure and saturating water vapor pressure in the same air
      pressure
2.) Example of Calculation of usage:
      Situation: 1 cubic meter air, relative humidity is 90%, surrounding temperature is 25℃ 
      The saturating pressure of water in 25℃ (298K) is 3169 Pa
      Current water vapor pressure
      = saturating water vapor pressure* relative humidity
      = 3169Pa*90%
      =2852.1Pa
      Ideal gas equation:
      n=PV/RT=2852.1×1/(8.31×298)=1.152(mol)
      [n is mole number,P is current pressure,V is volume,R ideal gas constantT is Kelvin scale(Celsius degree plus
      273)]
      1.152 mol water is 20.73g。
      That mains when the relative humidity and temperature are constant,the water in 1 cubic meter air is constant,we
      get the table in following:

T
relative
Humidity
0℃
5℃
10℃
15℃
20℃
25℃
30℃
100%
4.71
6.63
9.41
12.77
17.28
23.13
30.21
90%
4.31
6.12
8.41
11.55
15.55
20.73
27.52
80%
3.87
5.41
7.55
10.31
13.81
18.45
24.41
70%
3.40
4.69
6.57
8.92
12.21
16.17
21.52
60%
2.91
4.01
5.68
7.71
10.44
13.94
18.55
50%
2.44
3.44
4.75
6.44
8.67
11.48
15.34
40%
1.94
2.81
3.99
5.17
7.01
9.23
12.35

Table of the water content in 1 cubic meter air in some general temperature and relative humidity(unit:gram)
 
2、What courses the humidity?
When the container is transferred from high temperature area to low temperature area, the water in the container is constant. However, with the decreasing of temperature, the saturating pressure will be lower, according to the equation of relative humidity, the saturating pressure decreases, the relative humidity will increase. For example, 100% relative humidity, 30℃, the saturating water vapor content is 30.21 g/ m3, when the surrounding temperature change to 20℃, according to the table, the saturating water vapor content is 17.28 g/ m3, another 2.93 g/ m3 will be liquid water. If using the 20 feet container which has 31.5 m3, 31.5 m3×(30.21g/ m3-17.28 g/ m3)=407.295g will be liquid water, this will affect the production in the container.
 
 
3、How to reduce the humidity:
According to the equation of relative humidity, there are two variable, saturating water vapor pressure which is controlled by temperature and current water vapor pressure which is depend on the water vapor in air, so if we want to control the humidity just absorb the water vapor
 
4、Calculation of the time that our production can keep the container low humidity:
Example: 30℃, 100% relative humidity, water vapor content is 30.21 g/ m3, 20 feet container is about 31.5 m3, but 80% is production, so the water vapor is 31.5*30.21*20%=190.323g. Although the container is nearly a sealing area, it cannot avoid the exchange of air. Assume there is 150% (190.323*150%=285.4845g) air exchange every day, and our production can absorb all the water vapor.
Our desiccant can absorb 200% weight of itself, the weight of one container desiccant is 1 kg, can absorb 2 kg water vapor. Almost one 20 feet container use 6 desiccants, then 6*2.0=12kg water vapor can be absorbed.
Hence, we get the time is (12000-190.323)/285.4845=41.37day.
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